1. Field of the Invention
The present invention relates to the art of processing polymer compositions and specifically to a method of producing remoldable self-supporting thin panels consisting essentially of a polymer/filler composition in which the polymer consists, at least in part, of a thermoplastic polymer having a softening temperature of above about 150.degree. C. and wherein the filler consists, at least in part, of an essentially thermosensitive particulate organic material
2. Prior Art
Thin panels of this type are known per se and, typically, have a thickness between 1 and 5 mm, preferably between 1.5 and 3 mm. They are "self-supporting" in the sense of maintaining their shape under loads typical of their intended end use, such as when used as furniture components or as shaped panels in automobiles, e.g. as inner door casings; furthermore, they are "remoldable" in the sense that they can be shaped into most any required three-dimensional shape when heated or reheated to a thermoplastic state and processed in a conventional molding press, e.g. as disclosed in U.S. Pat. Nos. 4,243,456 and 4,328,067 with or without concurrent coating by lamination.
Such panels, in addition to their remoldability, must have a substantial mechanical strength combined with low weight while consisting essentially but of low-cost components These somewhat contradictory requirements can be met by using polymer/ filler compositions in which the polymer constituent imparts the required thermoplasticity combined with high mechanical strength (tenacity) while the filler should be capable of being used in substantial proportions, say 30 to 70 % by weight of the total composition, in combination with the polymer without impairing its essential characteristics and without adding much in terms of weight and costs
Particulate, i.e. comminuted, wood of the type known as "wood flour" or "saw-dust" and other organic substances preferably of the cellulosic type are known to meet these requirements, notably when used with high tenacity polymers, such as typically polypropylene of the homo- or heteropolymeric and frequently isotactic kind. However, molecular structures that yield high tenacity polymers tend to have relatively high "softening temperatures", i.e. the temperature at which they can be processed in their thermoplastic state, and typical processing temperatures of such polymers are in the order of 180 to 220.degree. C. Organic fillers of the cellulosic type, on the other hand, are not stable when exposed to such temperatures in that they tend to decompose at these temperatures and are termed "thermosensitive" herein for short to generally refer to organic fillers that would otherwise be suitable or desirable for use in production of panels because they meet the above aims but are degraded or decomposed at the processing temperatures dictated by the high tenacity polymers.
As is known, e.g. from U.S. Pat. No. 4,228,116, thermosensitive cellulosic fillers, such as saw dust, can be used for production of panels by the above described method if, prior to extrusion, an aggregated or agglomerated mixture ("premix") of particulate polymer and particulate filler is formed and if such mixture is introduced into the extruder. If the step of preparing the premix is omitted, the resulting extrudate tends to have insufficient coherence and/or has an uncontrolled cellular structure caused by voids that are formed due to inclusion of vapor bubbles of gaseous decomposition products including water that is strongly absorbed or chemically bound to the molecular structure of the organic filler within the polymer matrix around the filler particles.